In general, in a hydraulic system applied to an excavator or the like, a one or two stage relief value having a set pressure is installed so that the pressure of the hydraulic system can be maintained at a constant level to drive a hydraulic actuator or the like. Such a relief valve is used to perform a pressure boosting function of increasing the set pressure of the relief value upon the selection of the function by an operator depending on the work conditions. That is, the set pressure of the relief valve is boosted temporarily by the operator so that a torque of a force of the hydraulic actuator (e.g., a boom cylinder) can be increased.
In the meantime, the excavator allows a high load to occur during the work so that there frequently occurs a case where a hydraulic fluid discharged from a hydraulic pump is relieved. In this case, when the set pressure of the relief valve is boosted, the discharged hydraulic fluid is not relieved so that a loss of the hydraulic fluid relieved can be reduced. On the other hand, an operator suffers from an inconvenience of carrying out the pressure boosting function depending on the work conditions during the work, and thus such a hydraulic system is practically not applied to the equipment.